### ASA 126th Meeting Denver 1993 October 4-8

## 2aUW1. Low-frequency time-dependent scattering from evolving bubble
plumes: Theory and numerical results.

**Robert F. Gragg
Daniel Wurmser
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*Naval Res. Lab., Code 7144, Washington, DC 20375-5350
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This work advances the capability for modeling the scattering from
near-surface bubble plumes beyond the static approximation. Plumes were modeled
as evolving structures having finite lifetimes (~10 s) with distinct growth and
decay stages. The scattering response of such a plume was formulated in terms
of the theory of linear time-varying (LTV) filters. Complex scattering
amplitudes within the frequency band of interest (50--500 Hz) were computed
using either partial wave analysis [for (hemi-) spherical shapes] or boundary
integral equation (BIE) methods (for more general shapes). For cw inputs,
groups of pseudo-Doppler side peaks were observed in the scattered spectra as a
result of the time evolution of the target's internal resonances. Several
broadband source waveforms were considered to assess their effectiveness as
experimental probes for observing transient near-surface plumes in the field.
[Work supported by ONR, Code 4530.]